Small Rocky Worlds by the Billions?

byPaul GilsteronMay 31, 2008

My local paper is running a story on page 11A entitled “Astronomers Report Earth-like Planet.” It’s a tantalizing headline, but obviously one that bears further investigation. For what’s being reported here is background information on one of the 45 planets — I should say ‘candidate’ planets — recently discussed at the Boston meeting of the IAU. These have been extracted from the HARPS planet survey, but we’ll probably have to wait until mid-June for further confirmation, which may well occur at the upcoming Extrasolar Super-Earths workshop in Nantes.

This would be an interesting world if things do play out, a rocky ‘super Earth’ just over four times as massive as Earth, and hence the smallest world yet in our attempt to find planets not so different from our own. If the press continues to generate a buzz about this, we should look at the contrast with the Gliese 581 story. There we wound up with two planets of astrobiological interest, one apparently on the inner edge of the habitable zone and probably across it, too hot for life, with another on the outer edge. The jury is out on both in terms of habitability, but the odds went down considerably as various teams ran the numbers.

But while Gliese 581 is an M-class dwarf, the HARPS survey has been looking at F, G and K-class stars, the latter two classes not much different from our Sun. If we were to find a rocky world in the habitable zone of one of these, we would be a step closer to an ‘Earth-like’ world than a hot, tidally locked planet in tight orbit around a red dwarf. No wonder the press is interested. But again, we’ll have to await confirmation and the inevitable follow-up studies to the Geneva team’s work, nor have I seen any verification of the McClatchy news story’s further claim that the potential new world orbits in the habitable zone of its star (see below).

Somewhat misleading headlines aside, what really came out of the Boston IAU session was the growing understanding of how frequently rocky worlds occur. Based on the recent findings, they could outnumber Jupiter-class planets by three to one. Sara Seager (MIT) is being widely quoted on this, including this from the McClatchy story:

“The mass of the planets and the sheer number of them represents a huge step toward finding planets of the Earth’s mass and ones that might be suitable for life as we know it. What amazes me is that these planets may be very, very common.”

No wonder Seager sees the HARPS windfall as “…the beginning of the detailed exploration of super-Earths.” Excitingly, we’re also looking at the growing possibility of finding such a world in transit. To my knowledge, the 45 HARPS planet candidates all orbit in less than fifty days (making the habitability question seemingly moot around F, G and K-class stars), with some in orbits as short as ten days. Bagging a transit to follow up the HARPS radial velocity studies becomes easier when orbits are close and frequent, and such a transit would provide information about the planet’s diameter, density and composition, not to mention allowing potential studies of its atmosphere. But unless HARPS has other planets up its sleeve, ‘habitability’ may not be a factor in the next headline.

Exoplanet discoveries is certainly on the rise. It’s very exciting everytime I learn of a new one, of which there’s too many for me to actually keep track of now. My money’s on the 55 Cancri system, but who knows… how accurate can we really be this far away when a few million miles makes the difference between icy tomb and convection oven?

Rick, that was quick! I had posted this one not more than a minute or two before receiving your comment. You’re so right about uncertainty in the hunt for true Earth analogues, and we’re probably going to go through a number of hits and misses before we can really sort out what’s what. Exciting times…

paul and rick,you know what will actually be the biggest thing of all?! when they find a true earthlike planet.i feel that that will really excite people and turn out to be a real boone for the space program.people will want to know more. people will want to go! and also it will be a tremendous boone for those of us interested in interstellar flight because undoubtedly,this place will be many light years distant! as i have said before…people will be screaming -why hasn’t MORE been done in space exploration! and these will be people who last week considered money spent on space “wasted” !? but anyhow,seriously,if it creates a new demand for research into things like propulsion systems etc then it is indeed a very very good thing! this is indeed an interesting subject from many different angles. but that is all i have to say or can say for now.thank you guys,your friend george

Four times as massive as the earth? Hrm, if it is the same density then it would have what? 2.5 times earth gravity?

Wondering what sort of ecosystem could develop under such conditions, if it were within the habitable zone.

Tall trees would be out, especially if it also had a dense atmosphere with strong winds. It might a world of worms, insects, and burrowing creatures with nothing ever poking its head more than two or three feet off the ground.

With such massive gravity it would require more energy to move. Which would require either eating more, or more efficient digestive processes for the animals of that world. If the capacity of the vegetation to store energy was similar to earth’s then the animal population versus the vegetation would be a lot smaller.

If any creature did stand up right then to be able to handle falling it would need to either have incredible reflexes, or be extremely sturdy. Even falling here on earth for us can be pretty painful. Any creature which had to deal with the threat of falling might need more brain matter than a similar creature here on earth in order to have those reflexes.

With a lower animal/insect population how would a carnivore type creature manage. Probably not very well. Unless it trapped its prey in some way it would use a lot more energy to chase down its prey than would be used in a similar situation here on earth. Yet it would still only get the same amount of energy from the kill. Though the prey would not be able to move as fast as similar prey here on earth so …. hrm. It also would be case that find suitable prey would take more time since their density per area would be less.

A world of worms and snakes where the animals possess incredible reflexes, yet are slow moving. I wonder.

Rick, I’m afraid there’s very little chance for habitability in the 55 Cancri system. Last November, a fifth planet, one of ~Saturn mass, was announced in the habitable zone. The only hope for habitability is for any possible moons, but I’m not holding my breath.

The subject of animal life on other planets intrigues the heck out of me. I became interested in the possible extremes of such animal life several years ago after I had a rather bazaar series of mental images of what animal life on other planets might be like. Basically, I still remember the general time frame wherein I had a “What if?” mental image of a 1,000 foot long snake-like ET animal that had a body made of flesh with the same elastic modulus as Kevlar from which bullet resistant vests are commonly made. I guess the mental image was a spin-off on the movie “Dune”. I began to think, “It would be a very bad day to have an surprise encounter a hungry flesh eating predator with the above characteristics.” or something along those lines.

Another body type that comes to mind is the Silicon based monsters in a movie Alien and its several sequels.

However, I would not mind finding some cute and cuddly type of animals similar to house cats or affectionate canines. I can just imagine a huge market for ET kitties in the year 2400 brought back from some of our nearby stellar neighbors. At least, I like to dream about such.

Just to improve your quantification. Terrestrial planets increase in radius at between 0.268-0.272 power of the mass (in Earth units) – thus a 4 Earth mass planet, made of the same stuff, is roughly 1.45 Earth radii, and the gravity 1.9 gee. The time of a fall is 0.73 times its 1-gee equivalent. I don’t think our reflexes would need to be much more “super” than current reflexes and most small creatures happily carry twice their body weight, especially if the atmosphere is somewhat denser. Bipeds, like us, might have issues, but that’s another issue.

@Rick: the HZ can actually be quite wide, for the earth it is estimated from about 0.95 AU (yes, we are getting close to the edge ;-) ) and at least 1.2 AU, possibly up to 1.5 AU.

@david lewis: assuming equal density, the gravity would be about 1.6 times earth’s (because gravity is linearly related to mass, but inverse to the *square power* of radius).
Besides, what kind of ecosystems and organisms you would find on a high-gravity world also depends on a host of other factors, such as atmospheric density. For sea-dwelling creatures the gravity would hardly matter. Ecosystem productivity does not primarily depend on gravity but rather on atmosphere, soil, light (amount ans composition), etc.
If the atmosphere were dense, plants could still grow tall (high pressure), apart from other mechanisms.

1.58-1.9, would still mean more energy expended by a predator, affecting what sort of ecosystem could develop and how viable it would be. Which leads to some interesting speculation.

As for habitability, for any single world I wouldn’t hold my breath either. But with billions of such worlds, one can hope. Even if worlds are not in the habitable zone but were in the past life might have adapted to conditions that would have prevented it from arising in the first place.

Further to Adam, Centavra and my own post, if we assume equal density of such a 4 Earth-mass planet, then both the radius and gravity wil be almost 1.6 times Earth’s.
However, if the density of such a planet were half as much, i.e. the radius twice Earth’s, gravity wouls be similar. That would imply quite a light planet though, with less iron, and more silicon.
Maybe, but this is pure speculation, we might expect such a giant light weight earthlike planet near a less (but still sufficiently) metal-rich star, or at least one with less iron.

I think the point here is that our discovery of larger rocky worlds using the radial velocity method is amazing in of itself. If we can find lots of worlds that size with that, just how many could we find using the more advanced methods with the Terrestrial Planet Finder in 15 years?

It’s only a matter of decades before we find a truly Earth-type world as the post states. Might we find evidence of mind-brothers also?

The many-celled animals of the SuperEarth will need more time to develop, than the same animals on the Earth. For such beginnings, atmosphere must have oxygen. Without oxygen, exist could only simple and one-celled animals.
Let suppose, that atmosphere and ocean on the SuperEarth has four times bigger mass than on the Earth. After emergence life on the SuperEarth, about 4.8 billion years later, oxygen in its atmosphere will appear.
Why oxygen will emerge the 4.8 billion years later? It is because – a surface of the SuperEarth is 2.52 times bigger than a surface of the Earth. A one square unit of the SuperEarth has 1.59 times more atmosphere and water than Earth. The appearance of oxygen in atmosphere of the SuperEarth need 1.59 times more duration, than for appearance of oxygen on the Earth. We know, more than 3 billion year Earths plants oxidized iron in the ocean.

A couple of Points:
i) In response to David Lewis’s speculations on what surface terrestrial life would be like, if I remember from the articles I read on bio-mechanics (and I’m rusty on this) animals scale in direct proportion to the gravity. So the animal ecology could look like Earth’s only half the size. Trees would probably respond to high winds by becoming more flexable like palms. The big difference would be that their evapotranspiration limit would be cut in half from 400 feet to 200.

ii) I took a look at the titles of papers to be presented at the Extra-solar Super-Earths workshop and they look fascinating. I’d be very interested in reading their contents if anyone can direct me to them.

I’m not sure the energy requirements are going to scale linearly with the gravity: after all, inertia is not affected by gravity. Movement in the horizontal plane is thus unaffected. It still takes the same amount of energy to accelerate the same amount of mass. Maybe there will be some mass increase of the animals, but that may be offset by skeletal structure which may still allow some pneumaticity despite the requirements for the structure to be stronger.

In fact, because of the higher structural requirements on a high gravity planet, you may well end up with faster animals…

Abstract: To understand the formation and evolution of solar-type stars in the solar neighborhood, we need to measure their stellar parameters to high accuracy. We present a catalogue of accurate stellar parameters for 451 stars that represent the HARPS Guaranteed Time Observations (GTO) “high precision” sample. Spectroscopic stellar parameters were measured using high signal-to-noise (S/N) spectra acquired with the HARPS spectrograph. The spectroscopic analysis was completed assuming LTE with a grid of Kurucz atmosphere models and the recent ARES code for measuring line equivalent widths.

We show that our results agree well with those ones presented in the literature (for stars in common). We present a useful calibration for the effective temperature as a function of the index color B-V and [Fe/H]. We use our results to study the metallicity-planet correlation, namely for very low mass planets.

The results presented here suggest that in contrast to their jovian couterparts, Neptune-like planets do not form preferentially around metal-rich stars. The ratio of jupiter-to-neptunes is also an increasing function of stellar metallicity. These results are discussed in the context of the core-accretion model for planet formation.

dad wrote”our discovery of larger rocky worlds using the radial velocity method is amazing in of itself. If we can find lots of worlds that size with that, just how many could we find using the more advanced methods with the Terrestrial Planet Finder in 15 years?
It’s only a matter of decades before we find a truly Earth-type”

Good news. If KEPLER launches Feb ’09 as scheduled, we could find several Earth sized worlds by 2011-12. It needs a couple orbit ‘years’ to confirm a discovery.

TPF is not funded and will suffer delays (Kepler is several years ‘late’) after initial approval. Who knows when it or its analog will launch.

Formation and accretion history of terrestrial planets from runaway growth through to late time: implications for orbital eccentricity

Authors: Ryuji Morishima, Max W. Schmidt, Joachim Stadel, Ben Moore

(Submitted on 10 Jun 2008)

Abstract: Remnant planetesimals might have played an important role in reducing the orbital eccentricities of the terrestrial planets after their formation via giant impacts. However, the population and the size distribution of remnant planetesimals during and after the giant impact stage are unknown, because simulations of planetary accretion in the runaway growth and giant impact stages have been conducted independently.

Here we report results of direct N-body simulations of the formation of terrestrial planets beginning with a compact planetesimal disk. The initial planetesimal disk has a total mass and angular momentum as observed for the terrestrial planets, and we vary the width (0.3 and 0.5AU) and the number of planetesimals (1000-5000).

This initial configuration generally gives rise to three final planets of similar size, and sometimes a fourth small planet forms near the location of Mars. Since a sufficient number of planetesimals remains, even after the giant impact phase, the final orbital eccentricities are as small as those of the Earth and Venus.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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